Many present electronic systems incorporate cooling provided by brushless DC motor-based fans. These motors include control electronics which turn the coils of the motor on and off to get the rotor to spin. Many present fans have four wires accessible to the end user—a positive rail, a negative rail, a tachometer output which indicates speed, and a fourth wire for controlling the speed. These types of motors are very common as four wire high frequency fans that can be found in personal computers.
Controlling the speed of these types of brushless DC motors requires some way to slow the rate of rotation. This is often accomplished in one of two ways: (1) varying the DC voltage on the positive rail, which controls the amount of energy in the stator and thereby the speed of the rotor, or (2) by Pulse Width Modulating (PWM) the positive rail at its maximum voltage, which controls the amount of time the stator is energized and thereby the speed of the rotor. Many applications use PWM because of its ease of implementation from both a silicon and external circuit perspective.